Electrical resistor and method of making the same



Oct. 23, 1962 H. G. BICKFORD 3,060,063

ELECTRICAL RESISTOR AND METHOD OF MAKING THE SAME Filed June 17, 1960 INVENTOR. HARRY G.- BICKFORD ilk 1% ATTORNEY United States Patent 3,060,063 ELECTRICAL RESISTOR AND METHOD OF MAKING THE SAME Harry G. Bickford, Lafayette Hill, Pa., assignor to International Resistance Company, Philadelphia, Pa. Filed June 17, 1960, Ser. No. 36,977 6 Claims. (Cl. 117-217) The present invention relates to an electrical resistor and method of making the same. More particularly, the present invention relates to a protective coating for a metal film resistor, and the method of applying the coating to the resistor.

It has been found that certain thin metal films, and particularly chromium films, make good electrical resistors. To obtain the best quality metal film resistors, the metal films should be obtained by exposing a substrate of an electrical insulating material to the vapors of the metal which are evaporated from the metal in a vacuum. Such a method of forming an electrical resistor is described in United States Letters Patent No. 2,522,272 of S. A. Johnson et al., issued September 12, 1950, entitled Apparatus for Forming Metallic Films on Tubular Carriers, and United States Letters Patent No. 2,847,325 of J. Riseman et a1., issued August 12, 1958, entitled Apparatus and Method for Evaporating Films in Certain Types of Electrical Components. i I

However, such thin metal resistor films are subject to failure when the resistor is operated with the metal film exposed to the atmosphere. It has been found that such failure is caused by an electrolytic action brought about when the water vapor in the atmosphere contacts the resistance film which is under an electric load. Therefore, to prevent such failures of the metal film resistors it is desirable to protect the metal films from the moisture in the atmosphere.

It is an object of the present invention to provide a novel electrical resistor.

It is another object of the present invention to provide a novel protective coating for a metal film electrical resistor.

It is a further object of the present invention to provide a method for coating a resistor with a protective coating.

Other objects will appear hereinafter.

For the purpose of illustrating the invention there is shown in the drawing a form which is presently preferred; it being understood, however, that this invention is not limited to the precise arrangements and instrumentalities shown.

The drawing is a section view of an electrical resistor of the present invention.

Referring to the drawing, the electrical resistor of the present invention is generally designated as 10. In the illustrated embodiment the resistor 10' comprises a substrate 12 of an electrical insulating material, such as a plastic, ceramic, or glass. A thin film 14 of a metal having electrical resistance properties, such as chromium, is coated on the surface of the substrate 12. The metal resistance film 14 is preferably coated on the substrate 12 by exposing the substrate to the vapors of the metal which are evaporated fro-m the metal in a vacuum in the manner described in United States Letters Patents Nos. 2,522,272 and 2,847,325. Terminal caps 16 of electrically conductive metal are mounted on the ends of the substrate 12, and electrically contact the resistance film 14. Lead wires 18 of electrically conductive metal are secured to and project from the terminal caps 16. A protective covering 20 of the present invention is provided on the exposed portion of the resistance film 14 between the terminal caps 16.

The protective covering 20 of the present invention is "ice of an inorganic material which is insoluble in water, and which is impermeable to water vapor. In addition, the protective covering 20 is of a material which is compatible to the material of the resistance film 14 so that the protective covering 20 does not adversely affect the underlying resistance film 14. It has been found that the inorganic material which is most compatible with the metal of the resistance film 14 is a salt of the same metal as that of the resistance film. It has been found that for a resistance film 14 of chromium, a protective covering 20 of an anhydrous salt of a chromium halide selected from the group consisting of chromic chloride and chromic fluoride is both compatible with the chromium resistance film, and is insoluble and impermeable to water.

To form a Water impermeable film of either chromic chloride or chromic fluoride on the metal film resistor, the resistor 10 is placed in a hermetically sealed chamber. The chamber is then evacuated to a pressure suitable for evaporating the chromium halide coating material, approximately 4 to 5 10- milimeters of mercury. Also within the coating chamber is a supply of the solid salt of the chromium halide which is contained in means for heating the salt. When the coating chamber is evacuated to the proper pressure, the chromium halide salt is heated to a temperature of at least 325 C. to evaporate the salt. The vapors from the chromium halide salt diffuse in all directions from the source. The electrical resistor 10 to be coated is positioned in the coating chamber so as to be in the path of the chromium halide vapors. The vapors of the chromium halide which contact the resistance film 14 of the resistor 10 condense on the resistance film 14, and thereby form a protective coating of chromium halide around the resistance film. During the coating of the resistor 10, it is desirable to rotate the resistor about its own longitudinal axis to coat the entire surface of the resistance film 14. In order to achieve strong adhesion between the protective covering 20 and the resistance film 14, it has been found desirable to heat the resistor 10 to a temperature of not more than 60 C. while applying the protective coating 20 to the resistance film.

During the evaporation of the chromium halide, care should be taken not to heat the chromium halide salt high enough to cause excessive decomposition of the chromium halide salt. The decomposition of the chromium halide salt generates either chlorine or fluorine gas which can contaminate the electrical resistors being coated, and therefore must be removed from the coating chamber. The formation of the halide gas also increases the pressure within the coating chamber, and thereby may adversely alfect the coating process. Although the means for evacuating the coating chamber, such as a vacuum pump, will remove the halide gas to prevent contamination and variation in pressure, such halide gases are corrosive and may damage the evacuating means. Therefore, unless there is used special evacuating means which is not adversely affected by the corrosive action of the halide gases, care should be taken to keep the decomposition of the chromium halide salt at a minimum.

The chromium halide protective coating of the present invention is not only compatable with a chromium resistance film so that it does not adversely affect the chromium resistance film, but the chromium halide protective coating is also highly impermeable to water vapor. To test the affectiveness of the chromium halide protective films of the present invention, resistors having the chromic chloride protective film of the present invention and resistors not having such a protective covering were immersed in water, and a current was passed through the resistance films. The current passed through the resistance film of the resistors was suificient to produce a power dissipation of approximately 1 watt. The test was carried out until the resistors failed. The resistors not having the chromic chloride protective covering of the present invention failed within 5 to 8 seconds. However, the resistors having the chromic chloride protective covering of the present invention lasted for over 20 hours. When the resistors having the chromic chloride protective covering of the present invention which failed were examined, it was found that all of the failures were caused by damage to the substrate, and not to damage to the resistance film. Therefore, the failure of the resistors having the chromic chloride protective coating of the present invention was not caused by water penetrating the protective covering of the present invention. Similar results were obtained with a protective covering of a film of chromic fluoride, Although the resistor 19 is shown and described as having a resistance film coated on the outer surface of a solid cylindrical substrate, it should be understood that the protective covering of the present invention may also be applied to a resistance film coated on a thin fiat substrate, or a substrate of any other shape.

The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification as indicating the scope of the invention.

I claim:

1. An electrical resistor comprising a resistance film of chromium coated on a substrate of an electrical insulating material, and a protective coating covering said chromium resistance film, said protective coating being an anhydrous inorganic salt of chromium selected from the group consisting of chromic chloride and chromic fluoride.

2. An electrical resistor in accordance with claim 1 in which the protective covering comprises a layer of anhydrous chromic chloride.

3. An electrical resistor in accordance with claim 1 in which the protective covering comprises a layer of anhydrous chromic fluoride.

4. A method of forming an electrical resistor comprising coating a substrate of an electrical insulating material with a film of chromium, and applying a layer of an anhydrous inorganic salt of chromium selected from the group consisting of chromic chloride and chromic fluoride over the chromium film.

5. The method in accordance with claim 4 in which the anhydrous inorganic salt is a solid chromium halide selected from the group consisting of chromic chloride and chromic fluoride, and is applied by evaporating the solid chromium halide in a vacuum and condensing the chromium halide vapors on the resistance film.

6. The method in accordance with claim 5 in which the chromium film is heated while applying the chromium halide layer thereon.

References Cited in the file of this patent UNITED STATES PATENTS 

1. AN ELECTRICAL RESISTOR COMPRISING A RESISTANCE FILM OF CHROMIUM COATED ON A SUBSTRATE OF AN ELECTRICAL INSULATING MATERIAL, AND A PROTECTIVE COATING COVERING SAID CHROMIUM RESISTANCE FILM, SAID PROTECTIVE COATING BEING AN ANHYDROUS INORGANIC SALT OF CHROMIUM SELECTED FROM THE GROUP CONSISTING OF CHROMIC CHLORIDE AND CHROMIC FLUORIDE. 